It is well known in the electrical packaging arts that certain electrical components, for example, electrolytic capacitors, electrochemical cells, batteries of electrochemical cells, and double layer capacitors, can produce gases during continued operation. The pressure of those gases can rupture or burst a package. In order to avoid undue pressure increases within packages containing such components, it is desirable that gases generated within the package escape.
Packages incorporating membranes or plugs that respond to excessive internal pressures are known. Those packages may incorporate a membrane that ruptures or a plug that fuses or is expelled from a package in response to excessive internal pressure. These "one time" pressure relief mechanisms are extremely undesirable in many applications. Once a membrane is ruptured or a plug is melted or expelled, the electrical component within a package is exposed to the ambient. Essential fluids within the package can escape and undesired fluids, such as oxygen and water, can enter the package to cause or accelerate corrosion or other performance degradation mechanisms.
A porous diaphragm may also be used to release internal pressure. In U.S. Pat. No. 3,524,112, an electrolytic capacitor incorporating a membrane of rubber or neoprene having a diameter of 1 to 5 millimeters and a thickness of 0.5 to 2 millimeters is described. The membrane permits hydrogen diffusion out of the capacitor while preventing escape of the electrolyte. That patent also describes an electrolytic capacitor casing having a groove. A polycarbonate foil having a thickness of 0.02 millimeters is wrapped around the casing over the groove. When pressure within the capacitor becomes too large, an edge of the foil is temporarily displaced from the package surface and allows gas to escape. In addition, the foil permits hydrogen to diffuse out of the capacitor. The displacement of the foil means that foreign matter can enter the package and electrolyte can escape from the package.
A battery vent employing a membrane that is permeable to oxygen and hydrogen but that is impermeable to the sulfuric acid electrolyte is described in U.S. Pat. No. 3,909,302. The membrane, having a thickness of 0.1 to 0.8 millimeters, is microporous polytetrafluoroethylene. The membrane holds back the liquid battery electrolyte while permitting the gases to escape from the battery so long as the liquid electrolyte does not occlude the membrane.
Double layer capacitors employ electrolytic elements comprising activated carbon and an electrolyte, typically sulfuric acid. The carbon contains many pores, producing a very large surface area for charge storage. Two of these electrolytic elements are brought together with an intervening ion conducting, electrically insulating separator to form a capacitor element. For increased operating voltage, many of the capacitor elements are stacked with intervening electrically conductive plates as terminals of the individual capacitor elements. See U.S. Pat. No. 4,683,516, the disclosure of which is incorporated herein by reference.
Double layer capacitors have an extremely high energy storage density. Capacitances of one farad and more with essentially unlimited voltage capabilities can be produced by connecting double layer capacitor elements in parallel and series. Double layer capacitors can generate carbon dioxide during operation. At least some of the carbon dioxide is believed to be reactively produced from oxygen that has been adsorbed on the carbon in the capacitor elements and from oxygen that may leak into the package. Therefore, venting of carbon dioxide and exclusion of external oxygen from a packaged double layer capacitor has been recognized as an important packaging goal.
Even though the total amount of carbon dioxide generated in a double layer capacitor is relatively small, the small volume of a typical double layer capacitor package that is not occupied by solid materials, i.e., the "empty volume" means that relatively large pressures can build up from the generated gas. For example, a package having an "empty volume" of one cubic centimeter may be able to withstand an internal pressure of ten atmospheres without bursting. If the packaged capacitor has a design lifetime of ten years, the average gas generation rate cannot exceed about 3.times.10.sup.-6 moles per year without bursting the package before the end of its design lifetime.
In double layer capacitors, as in other electrical components that generate gases, a displaceable, fusible, or rupturable plug or a similar pressure-relief mechanism that gives access, even temporarily, to the inside of the package is highly undesirable. That access permits the undesirable entry of deleterious materials into the package, such as oxygen in a double layer capacitor, and allows useful materials, e.g., electrolyte, to escape, seriously degrading performance. In double layer capacitors, electrolyte loss, measured by the decrease in the weight of the packaged component, causes a catastrophic decline in capacitance and increase in equivalent series resistance (ESR). (The ESR is a measure of the degree of difficulty of charging and discharging a capacitor. Since a high ESR means a capacitor has failed, ESR is a particularly sensitive indicator of a capacitor's condition.)
In the inventor's U.S. Pat. No. 4,992,910, the disclosure of which is incorporated herein by reference, a package for an electrical component that generates a gas during operation is described. That package includes a metal container and a means for selectively venting the gas generated within the package. That means for selectively venting may be a selectively permeable body disposed within and closing an opening in the metal container that permits a gas generated with the package to escape from the package while retaining within the package at least one desired fluid different from the generated gas so that premature failure of the electrical component and/or the package is prevented. Generally, the permeable body is a polymeric material, such as silicone rubber, polypropylene, natural rubber, butyl rubber, polytetrafluoroethylene, or polyethylene. While the package described in U.S. Pat. No. 4,992,910 is effective in preventing premature failure of a packaged component and the package, the package requires relatively complex manufacturing techniques. The selectively permeable material must be attached in some way, for example, with an adhesive, to the metal container. One of the best performing selectively venting polymeric materials is polytetrafluoroethylene, sold by Dupont under the trademark TEFLON. However, polytetrafluoroethylene is notoriously difficult to attach to a metal.
Accordingly, it is desirable to provide a packaged electrical component including a package that is easily manufactured wherein the electrical component generates a gas during operation and the package permits the generated gas to escape from the package but retains desired fluids within the package and that prevents undesirable materials from entering the package.